As one of typical heating devices, a fixing device for use in an electrophotographic printing device such as a copying machine, a printer, etc., is arranged so as to fix a toner image by applying heat and pressure using a pair of rollers (a fixing roller and a pressure roller) which are disposed so as to press one another (heat roller fixing system). Namely, the fixing roller on the side facing the toner image is heated to a predetermined temperature (fixing temperature) by heating means such as a halogen heater, etc., provided inside the fixing roller.
In recent years, as disclosed in Japanese unexamined patent application 2000-338818 (Tokukai 2000-338818; published on Dec. 8, 2000), the structure wherein the surface of the pressure roller is externally heated using the external heating means for pressure roller such as an external heat roller, etc., in contact with the pressure roller (external roller heating system) has been proposed. The fixing device that permits a shorter warm-up time offers the effects of reducing power consumption, increasing the speed as well as suppressing the sheet from being curled.
This fixing device will be explained in reference to FIG. 4.
FIG. 4 shows schematic structures of essential parts of a fixing device using a conventional external roller heating system.
As shown in FIG. 4, the fixing device of the external fixing system includes a fixing roller 131, a pressure roller 132, an external heat roller 133, heater lamps 134, 135 and 136, temperature sensors 137, 138 and 139, a cleaning roller 140 and a temperature control circuit (not shown).
The external heat roller 133 serves as external heating means. The heater lamps 134 and 135 serve as a heat supply for the fixing roller 131. The heater lamp 136 serves as a heat supply for the external heat roller 133. The temperature sensors 137 and 138 serve as temperature detection means for detecting the temperatures of the fixing roller 131, and the temperature sensor 139 serves as temperature detection means for detecting the temperatures of the external heat roller 133. The temperature control circuit serves as the temperature control means.
In the following, each of the foregoing parts will be explained in details.
Firstly, the heater lamps 134 and 135 are provided inside the fixing roller 131, and the heater lamp 136 is provided inside the external heat roller 133. Then, when fed with electric current by a temperature control circuit, the heater lamps 134, 135 emit light at the infrared wavelengths with a predetermined heat distribution. As a result, the inner surfaces of the fixing roller 131 and the external heat roller 133 are heated respectively.
The fixing roller 131 is heated to a predetermined temperature (for example, 190° C.) by the heater lamps 134 and 135 to heat the recording sheet P having formed thereon an image of unfixed toner T which passes through a fixing nip section Y of the fixing device.
The heat roller 131 is made up of a core metal 131a and a releasing layer 131b. This core metal 131a forms the main body of the heat roller 131. The releasing layer 131b is provided on the outer surface of the core metal 131a to prevent the toner T on the recording sheet P from offsetting.
The core metal 131a is preferably iron, stainless steel, aluminum, copper, or an alloy of these metals. Specifically, the metal 131a is, for example, stainless steel or carbon steel. Here, the core metal 131a is iron (STKM) and has a diameter of 40 mm and a thickness of 0.4 mm to reduce thermal capacity.
Here, it is preferable that the releasing layer 131b be made of fluororesin, such as PFA (perfluoroalcoxyalkane; a copolymer of tetrafluoroethylene and perfluoroalkylvinylether) or PTFE (polytetrafluoroethylene); silicone rubber; fluororubber; or similar material.
The pressure roller 132 includes a metal core 132a with a diameter of 40 mm, made of iron steel, stainless steel, aluminum, etc., and a heat resistant elastic layer 132b made of foamed silicone rubber on the outer surface of the metal core 132a. On the surface of the heat resistant elastic layer 132b, further formed is a releasing layer 132c made of fluororesin as in the case of the fixing roller 131. The pressure roller 132 is pressed to the fixing roller 131 by a spring or other pressure member (not shown) with a force of, for example, 274 N. Then, the fixing nip section Y, about 6 mm wide, is formed between the pressure roller 132 and the fixing roller 131.
The external heat roller 133 is heated to a predetermined temperature (200° C., for example,) by the heater lamp 135 (internal heat supply). This external heat roller 133 is provided on the upstream side of the fixing nip section Y, and placed in press contact with the pressure roller 132 with a predetermined press contact force. This external heat roller 133 forms a heating nip section Z (1.5 mm, for example) with the pressure roller 132.
The external heat roller 133 includes a cylindrical metal core material 133a made of aluminum with a diameter of 15 mm and a thickness of 1 mm, and a fluorocarbon resin layer made of synthetic resin which shows excellent heat resistance and mold releasing property as a heat resistant releasing layer 133b formed on the metal core material 133a. 
The cleaning roller 140 is provided for removing toner particles, paper particles, etc. from the pressure roller 132, preventing the external heat roller 133 from being contaminated. The cleaning roller 140 is disposed on the upstream side of the heating nip section Z and presses the pressure roller 132 with a predetermined press contact force. Supported at the axis, the cleaning roller 140 is rotated by the rotation of the pressure roller 132. The cleaning roller 140 is a cylindrical core material.
The pressure roller 131 and the external heat roller 133 are provide with thermistors 137 to 139 as temperature detection means on their outer surfaces respectively. These thermistors 137 to 139 are provided for detecting the respective surface temperatures of the pressure roller 131 and the external heat roller 133. Based on the resulting temperature data as detected by the thermistors 137 to 139, the temperature control means (not shown) controls the conducting of current to the heater lamps 134 to 136 respectively, so as to maintain the respective surfaces of the pressure roller 131 and the external heat roller 133 at predetermined temperatures.
After heating the pressure roller 132 to a predetermined temperature (fixing temperature) by the external heat roller 133, an image of the toner T is heated (fixed) with applications of heat and pressure by making the recording sheet P having formed thereon an unfixed image of the toner T through the press contact region Y between the fixing roller 131 and the pressure roller 132.
The foregoing external roller heating system permits thermal energy to be supplied to the recording sheet P aggressively also from the side of the pressure roller 133 unlike the conventional heat roller fixing system.
With the resulting increase in thermal energy applied to the recording sheet P, it is possible to reduce the surface pressure (load) to be applied to the recording sheet P. Particularly, when adopting the external roller heating system to the conventional high speed apparatus (for example, with a copy speed of 25 sheets per minute in cross feeding of A4 size recording sheet) that requires heavy load due to a lack in thermal energy to be applied to the recording sheet P, the effects as achieved with a reduced surface pressure can be appreciated. That is, the fixing roller 131 can be made thinner and smaller, which in turn reduces the thermal capacity of the fixing roller 131, thereby realizing saving of energy with reduced warm-up time.
The fixing device of the external roller heating system, however, has the following problems.
{circle around (1)} The heat supply from the external heat roller to the pressure roller is dominated by the heat transfer by conduction at the heating nip section. The heating nit section, however, has a width of at most several mms, and therefore, an efficient heat supply system cannot be ensured with limited heating performances (heating temperatures).
{circle around (2)} Ac compared to the conventional heat roller fixing device without the external heat roller, the surface of the pressure roller becomes high, and an amount of heat radiated from the surface of the external heat roller increases, and besides that heat is radiated from the surface of the external of the external heat roller at the same time. Therefore, depending of the structures of the external heat roller (roller diameter, roller thickness, load, control temperature, etc.), an amount of heat radiated from the fixing device increased as compared to the case of adopting the heat roller fixing system, which results in an increase in overall power consumption (reduction in heat efficiency).
{circle around (3)} The respective surface conditions of the external heat roller and the pressure roller change as time passes, and it is therefore difficult to make the external heat roller to be driven by the pressure roller under stable conditions, which results in a problem that a slipping of the external heat roller is liable to occur. On the other hand, in order to ensure smooth rotations of the external heat roller under stable conditions, noise sound is liable to occur by forcing, for example, the gear to drive.
{circle around (4)} The external heat roller is contaminated by toner paper powders, etc., which is liable to cause the sheet to be contaminated. In response, cleaning means such as a cleaning roller, etc., may be provided on the upstream side of the external heating roller. However, by providing the cleaning means, the structure of the fixing device becomes complicated, and in the meantime, heat load, heat radiation occur, resulting in reduced heat efficiency.